A quad flat package or QFP is an integrated circuit comprising a rectangular or a square body, generally made of either plastic or ceramic material, from whose periphery gull wing shaped leads extend. QFP chip packages have gull wing type leads on four sides. As technology advances, chips become smaller in size and number of leads thereon becomes greater. As a result, size of leads and clearance there between has become smaller and smaller.
On the other hand, it is necessary for all leads on a chip to have proper clearance, not to be bent out of shape and to be coplanar. This means, when placed on a flat surface (for example, a printed circuit board), all leads should preferably be touching the flat surface or at least display a minimum clearance to allow proper soldering of the leads to the surface when required. QFP leads that lack the aforementioned cannot be properly soldered resulting in higher manufacturing costs and, if not detected early in the manufacturing process, even costlier failures in the field.
QFP chips are generally packaged in groups of 12 or more and shipped in standard trays to prevent damage of the very fragile leads. QFP chips are generally inspected inside their respective standard trays. In addition to facilitating handling (complete trays instead of individual devices), such an approach reduces the possibility of further lead damage by the inspection system as there is no device manipulation. QFP inspection consists of inspecting different lead characteristics, such as clearance between adjacent leads, bent conditions, overall lead position (geometry) and co-planarity.
One problem with QFP devices sitting in a tray pocket is that, due to lead occlusions, it is nearly impossible to scan lead tips with a single laser triangulation system oriented in a fixed direction. In fact, a typical laser triangulation system has to scan in two transversal axes, in parallel with lead main axes.
Prior art scanning systems display different scanning techniques as a solution to the above problem, such as (1) using two different laser triangulation systems (one for each of the two transversal directions and switching between them depending on a scanning direction) or (2) using a single laser triangulation system with a rotating head (and rotating the head as a function of the scanning direction) (see U.S. Pat. No. 5,406,372). Another technique used is a single laser triangulation system oriented at 45° of the two transversal directions to scan in one direction only.
All of the aforementioned scanning techniques have certain drawbacks, such as high costs (when two laser triangulation systems or a rotating head is used), accuracy problems (when a rotating head is used) and reduced efficiency in case of long leads with short tips (QFP208 devices) (when a laser system oriented at 45° is used).
Besides the aforementioned scanning techniques, another prior art technique consists of scanning QFP devices in an upside-down position (also known as a “dead bug” position), however most tray designs cannot guarantee such a position for QFP devices. Moreover, placing QFP devices in a “dead bug” position inside a tray risks damaging their leads when stacking trays.